With recent rapid developments in electronic and information industries, there have been keen demands for further minituarization and high-integration in precision components, and super-precision positioning devices which can be used in nano-order (10−9 m-order) inspection or processing are required. Also, with developments made in applied technologies in medicine and biotechnological researches on controlling proteins and cells, there is a keen demand for precise positioning in finer regions of microscopic stages. Moreover, recently, together with demands for higher-precision and downsizing in objects of inspections, processing and measurement, there have been demands for reduction in size and weight of positioning devices and drive sources thereof.
Conventionally, as drive sources of positioning device, electromagnetic motors have been used. A positioning device using an electromagnetic motor, however, has not only problems of various disadvantages attributable to the electromagnetic motor but also many other problems such as a structural problem that such a device requires a reduction gear and a ball screw. It is difficult to obtain nano-level precision by such a device. Furthermore, such a structural disadvantage inevitably makes the occupied volume and weight large. In a case of positioning using an electromagnetic-motor type device, the higher-precision limit in positioning is 1 μm (1.0×1.0−6 m) even when a device which is considered as having relatively high precision is used. This can be said to be 1000 times coarser as compared with nano-order precision required in the industrial market. That is, it is very unlikely that such a positioning device using an electromagnetic motor can achieve nano-order precision positioning.
As a new type positioning device, a positioning device using an ultrasonic motor as its drive source is expected to replace conventional electromagnetic motor-type devices. Such an ultrasonic motor-type positioning device uses a principle of action that ultrasonic oscillation is converted into friction, in which no reduction gear or ball screw is required. Further, such a device is expected to have excellent properties such as being reduced in size and weight, high responsiveness, no operating noise, and high retention at down time. Thus, technology using an ultrasonic motor as a drive source has been attracting attention as enabling provision of a positioning device of very high precision. Many types thereof have been proposed and studies have been made on such a technology.
The general principle of ultrasonic motor is shown in FIG. 2. The ultrasonic motor comprises vibrator 1 and slider 2 (dynamic body). At least one part of the vibrator makes an elliptic motion by combination of expansion, contraction and flexion. For example, point p (the center of the left end surface of the vibrator) in FIG. 2(A) goes through four states (a) to (d) to thereby describe a trajectory as shown in FIG. 2(B) (x is a longitudinal axis of the vibrator and y is an axis vertical to the top and bottom surfaces of the vibrator.) To the part of the vibrator making an elliptic motion, generally a wear-resistant material is adhered to serve as fixed sliding member 3 (stator). The elliptic motion of the vibrator is transmitted via the stator to a slider, to serve as power driving the slider. In FIG. 2, the dynamic body is transferred downwards along guide 4 shown in the figure by repeated elliptic motion. In this example, although the dynamic body is driven in a straight line, it is possible to make a rotary motion if a circular dynamic body is prepared.
As described later, the vibrator can consist of a piezoelectric element. In this case, the scale of the elliptic motion of the piezoelectric element depends on the input voltage. That is, as typically shown in FIG. 3 (corresponding to FIG. 2(B). In FIG. 3, y axis of FIG. 2(B) is shown as a horizontal direction), the larger the input voltage, the larger the scale of the elliptic motion and the higher the speed at which the slider moves. On the other hand, the smaller the input voltage, the smaller the scale of the elliptic motion and the lower the speed at which the slider moves. For example, in a case where an ultrasonic motor is used for precisely positioning on an X-Y table or the like, it is necessary to decrease the moving speed as getting close to a target position so that it can gradually get closer to the target. In order to decrease the moving speed, it is necessary to reduce the input voltage.
On the other hand, the reason why drive power can be transmitted via the fixed sliding member to the slider is that the two members contact with each other. For this purpose, the stator is pressed against the slider at an appropriate pressure. If the pressure is too strong, however, the stator is pressed against the slider so much that the two members can never separate from each other even for a moment and by weak input voltage, the motor does not work at all. (This corresponds to a state that there is no horizontal amplitude in the elliptic motion of FIG. 3. In a conventional vibrator, vertical amplitude also becomes zero in such a state.) When the input voltage is gradually increases, the vibrator suddenly starts to work at a certain voltage (threshold voltage), and in a region exceeding the threshold voltage, an elliptic motion begins to occur.
Even if the pressing force is reduced to the minimum, the threshold voltage only becomes low but does always exist. Therefore, it is difficult to prevent the behavior of such a sudden actuation at a certain voltage (threshold voltage). That is, the relation ship between the voltage and the moving speed does not have proportional characteristics in a low voltage region and in addition, it forms a very precipitous curve line as shown in FIG. 4. Even a small fluctuation in the voltage leads to a significant change in the moving speed (nonlinearity of input-output characteristics).
In this way, in a case where precise positioning is conducted by using an ultrasonic motor, it is necessary to make the input voltage low. For the nonlinearity of input-output characteristics, control is difficult in a fine movement region.
As examples of conventional vibrators for ultrasonic motors, Japanese Patent No. 3311446 and Japanese Patent Application Laid-Open No. 2004-297951 (US patent application publication No. 2004/189155) disclose mechanisms where by exciting a rectangular stacked-type piezoelectric element to simultaneously make two kinds of oscillation, expansion-contraction and flexion, an elliptic motion is allowed to occur at a certain part of the vibrating body, and the elliptic motion is transmitted to the dynamic body (slider) so that the dynamic body can make a rotary motion or straight motion.
In Japanese Patent Application Laid-Open No. 2000-116162 (U.S. Pat. No. 6,218,769) and Japanese Patent Application Laid-Open No. 2005-65358, by stacking piezoelectric elements for exciting second flexural vibration and piezoelectric elements for exciting first stretching vibration respectively, the vibrator is excited to cause an elliptic motion.
In conventional vibrators for ultrasonic motors, however, nonlinearity of input-output characteristic is not overcome. Therefore, controllability in a fine movement region is low, which results in difficulty in precise positioning.
Moreover, in conventional vibrators for ultrasonic motors, a piezoelectric element forms electrodes on almost the whole surface of both sides sandwiching a plate-like element, as typically shown in Japanese Patent No. 2722211. For this feature, there is a problem that capacitance becomes large and an unnecessarily large amount of current flows, which leads to large burden on a driving power source. Furthermore, in Japanese Patent Application Laid-Open No. 2000-116162 (U.S. Pat. No. 6,218,769) and Japanese Patent Application Laid-Open No. 2005-65358 where, piezoelectric elements for exciting second flexural vibration and piezoelectric elements for exciting first stretching vibration respectively are stacked separately, it means that one half of the whole vibrator body causes flexural vibration and the other half causes stretching vibration. For this feature, the proportion of excitation in the whole elements is small, which results in small vibration amplitude of the vibrator and low efficiency in excitation of vibration.